A data storage device, for example hard disk drives (HDDs), conventionally uses Reed Solomon codes as the main codes to ensure the reliability of the information stored on the media in the presence of several distortions introduced by the recording process. These distortions may include:
1. Media noise
2. Intersymbol interference (ISI)
3. Intertrack interference (ITI)
4. Non-linearities (eg: NLTS, PE)
5. Electronics noise (AWGN).
Media noise is the dominant distortion on the recording channel, and its impact is expected to grow as channel density increases and the number of grains storing each channel bit decreases. For example, the ratio of user bits and grains and channel bits may be 1 user bit to 2 grains to 4 channel bits. Channels are currently designed with a certain degree of ISI introduced by the readback process that is mitigated by the equalizer, but typically, with no ITI. That is, the tracks are separated by enough distance that the readback signal is not influenced by the magnetization on the adjacent tracks. However, in the squeeze to higher densities, this may change. Non-linearities may also become more severe as densities increase, however the electronics noise may remain somewhat unaffected and may be of much lesser significance as compared to the other type of distortions.
The HDD recording channel have progressed from Reed-Solomon Codes to LDPC codes and using iterative decoders.
Low-density parity-check (LDPC) codes and iterative detectors are computationally intensive for implementation in a real (real-life) system. However, with the progression of the technology in implementing algorithms in silicon, together with complexity reduction of the detection/decoding algorithms, virtually all HDD recording channels are currently LDPC coded and iteratively decoded. However, the message passing algorithm used in the iterative decoder is not optimal.